1. Field of the Invention
The present invention relates to a linear motor provided with a rod-like stator having a drive field magnet extending in a uniform direction and a movable piece having an armature coil fitted around the field magnet.
2. Description of the Background Art
As shown in FIG. 12(A), conventional shaft type linear motor includes a rod-like stator 3 having a filed magnet 3M which is provided at its surface with N- and S-poles aligned alternately to each other in a uniform direction, and a movable piece 30 having an armature coil 31 arranged around a cylindrical member 33 which is made of plastics and fitted around the stator 3. The armature coil 31 is accommodated in a cylindrical movable piece yoke 32, and the plastic cylindrical member 33 is provided at its inner surface with a plurality of unillustrated guide projections which are in slidable contact with the field magnet.
The linear motor of the type, which includes the rod-like stator having the field magnet extending in a predetermined direction and the armature coil fitted around the field magnet as described above, can utilize the rod-like stator as a guide member for the movable piece, and it is not necessary to provide additionally large or complicated guide means for the movable piece, so that the structure is simple. By this and other reasons, the linear motors of the above type have been utilized in various kinds of instruments.
For example, in image forming apparatuses such as a copying machine and image reading apparatuses such as an image scanner, it has been attempted to use the linear motors of the above type as a drive source of a scanning optical system for a document image.
In the linear motors of the above type, however, when the movable piece moves relatively to the stator, change of a load, i.e., cogging generates due to change of a magnetic attraction force which is applied from the drive field magnet (i.e., field magnet for driving) to an end of the movable piece yoke. This prevents a smooth movement of the movable piece.
Meanwhile, a structure shown in FIG. 12(B) is also conventionally known. In this structure, an armature coil 21 of a movable piece 2 is opposed to a plate-like stator 1 having a field magnet 11, and rollers 23 provided at a movable piece yoke 22 roll on a stator carrying base 12. In the linear motor of the above structure, the movable piece yoke 22 is provided at its front and rear ends 221 and 222 in its moving direction with triangular projections in order to suppress change of a magnetic attraction force acting on the ends of the movable piece yoke 22.
In the linear motor of the flat type described above, however, the attraction force by the field magnet 11 acts on the movable piece yoke 22 in the same direction as the gravity, so that an excessively large load is applied to the portions movably carrying the movable piece 2. In addition to the above, the projections formed at the ends of the movable piece yoke 22 increase the whole area of the movable piece yoke 22, which further increases the attraction force acting on the yoke in the gravity direction, and thus the load further increases.
The linear motors of the shaft type also suffer from the following problem. In order to allow movement of the movable piece along the rod-like stator, there is arranged the plastic cylindrical member provided at its inner surface with a plurality of projections which are in slidable contact with the stator, and the armature coil is arranged around the cylindrical member, so that the cylindrical member operates as a kind of bearings. However, it is practically difficult to manufacture the movable piece provided with the cylindrical member having the projections in view of precision and others. Further, arrangement of the armature coil around the cylindrical member causes such disadvantages that the cylindrical member forms an obstacle, and that the cylindrical member excessively increases a distance between the armature coil and the field magnet, so that it is difficult to obtain an effective driving force from a magnetic flux of the field magnet.
In the linear motor of the flat type, a large space is required for arranging the armature coil in the movable piece, which increases the sizes of the movable piece. Also, arrangement of the movable piece guide outside the stator and movable piece requires a dedicated or additional space, so that the whole sizes are liable to be large. Further, due to the arrangement of the armature coil, it inevitably has a portion which does not contribute to the driving force.
Accordingly, it is an object of the invention to provide a linear motor, which includes a rod-like stator having a drive field magnet extending in a uniform direction, and an armature coil fitted around the field magnet and covered with a movable piece yoke (e.g., the armature coil is arranged inside a cylindrical movable piece yoke), and can operate stably and accurately.
Another object of the invention is to provide a linear motor, which includes a rod-like stator having a drive field magnet extending in a uniform direction and a movable piece being reciprocatable along the stator and having an armature coil, and allows a compact structure, a high utilizing efficiency of a magnetic energy, and stable and accurate operation.
Still another object of the invention is to provide an apparatus in which the above linear motor according to the invention allowing a compact structure, and efficient, stable and accurate operation is employed as a drive source for a member to be driven to move for achieving a compact structure and improving an accuracy.
Here, the above "member to be driven to move" is, for example, a carriage carrying a lighting lamp, mirror or the like to be moved for image scanning in an image scanning optical system of an image reading apparatus in a copying machine, image scanner or the like, or is a printing head, a writing unit or the like in a printer. The above "apparatus" is, for example, the image reading apparatus or the printer.